Provenance and age of igneous clasts in Mesozoic Murihiku conglomerates, Catlins Coast, New Zealand

Abstract

The provenance of the Murihiku Terrane has been investigated using petrography, geochemistry and geochronology on igneous, mainly plutonic, clasts found in rare conglomerates along the Catlins Coast, southeastern New Zealand. Late Triassic (Oretian) conglomerates at Roaring Bay and Middle Jurassic (Temaikan) conglomerates at False Islet, Waikawa, and Slope Point have been studied in detail. These are generally polymict, poorly sorted, and pebble to cobble (uncommonly boulder) grade. The mid-Jurassic conglomerates were deposited in a fan-delta to terrestrial setting. Rounded to subrounded volcanic clasts dominate with subordinate volcaniclastic, plutonic (c. 15%), and rare metamorphic clasts.

The 18 plutonic clasts geochemically analysed (whole-rock; ICP) range from quartz monzodiorite to alkali feldspar granite (59–79 SiO2 wt%) and generally have subduction-related arc geochemistries. The majority are I-type but a small number have weak A-type affinities, and one is strongly A-type. Three volcanic clasts were also analysed. Three of the 21 total clasts analysed have adakitic (HiSY) affinity. Subduction-related characteristics are seen such as negative Nb and Ta anomalies. Clasts from all the conglomerate horizons have petrographic similarities.

Zircons from 13 plutonic clasts have been dated by LA-ICP-MS yielding the following 208Pb-corrected concordia ages with 2σ errors. The main clast age cluster is broadly Middle Permian (258 ± 3 Ma, 263 ± 2 Ma, 266 ± 3 Ma, 268 ± 3 Ma, 269 ± 3 Ma, 270 ± 3 Ma, 272 ± 2 Ma). In addition there are clasts with younger Triassic (218 ± 2 Ma, 240 ± 3 Ma) and Jurassic (168 ± 2 Ma, 187 ± 2 Ma), and older latest Carboniferous (300 ± 3 Ma) and mid-Carboniferous (333 ± 3 Ma) ages. An igneous clast with an age that is penecontemporaneous with its depositional age was found in both a Middle Jurassic and Late Triassic conglomerate. It is possible that the False Islet, North Head, and Slope Point conglomerates have the same broad depositional age, and could represent the same stratigraphic level.

Many of the analysed clasts show similarities with units within the adjacent Median Batholith on the basis of their combined petrography, geochemistry and age characteristics. The Darran Suite shows an excellent match with the 168 Ma clast (Rakeahua Pluton and Hunter Intrusives), and the 218 Ma clast with Mistake Suite. The 300 Ma A-type alkali feldspar granite clast is comparable to the Freds Camp Pluton of the Foulwind Suite, and the 333 Ma clast may be from a younger Tobin Suite member such as the Lake Roxburgh Tonalite. The Permian cluster is difficult to correlate with New Zealand rocks. Alternatively, the source of these may extend to the voluminous Mesozoic to Late Palaeozoic batholiths that occur in formerly contiguous eastern Australia and West Antarctica.

This study has shown that conglomerates can be an important tool when investigating the provenance of a sedimentary tectonostratigraphic terrane. Conglomerate clasts provided hand specimen size samples of source rocks to the Murihiku Terrane. This size of sample allowed for much more extensive geochronology, petrography, and geochemistry analysis than could be otherwise carried out on detrital grains within the dominant finer grained sandstone lithologies. Conglomerate clasts therefore provide more robust evidence to any provenance determinations.